Types of power generation facilities include gas turbine facilities, boiler or steam turbine facilities, and combined cycle turbine facilities (also known as gas and steam-turbine plants). Steam flows available in the latter two, steam-cycle types of facilities typically are directed, after some treatment to adjust temperature, for use in a number of operations including, but not limited to, cold starts, warm starts, shutdowns, control of process steam extraction, low load operations, and adjustment of steam temperature for industrial applications.
A cold start occurs when a turbine is started after a sufficient period of time that the temperature of its components have fallen below a specified temperature, such as when the low pressure rotor bore metal is below 200 degrees Fahrenheit. Cold starts are common in facilities that supply electrical power to residential grids. This is because power requirements there are diurnally cyclical and some turbine units are routinely shut down during low power requirement periods, and are restarted when demand increases. Even for other, non-residential or community-supplying power generation plants, cold starts, as well as warm starts, shutdowns, low load operations, and/or other operations needing steam temperature control may be required under various circumstances. Thus, proper regulatable control of steam temperature for such operations is generally needed.
Further regarding a cold start, a proper gradual warm up of a cold turbine allows for adequate treatment of steam seals in the turbine, gradually warms other turbine components, and thereby avoids steam turbine thermal cyclic damage. Because thermal stresses upon the metal components of steam turbines are known to adversely affect these components, leading to increased repairs and down time, and to shortened turbine life, various approaches are employed to assure a sufficiently gradual increase in steam temperature during turbine cold starts.
A common approach to providing steam at a gradually increasing temperature is to incorporate and utilize an in-line attemperator in a steam line from which steam is thereafter directed to enter, warm up, and start up a turbine. An in-line spray-nozzle type attemperator provides water through one or more nozzles or other apertures into the steam line. The water mixes with the steam, largely vaporizes, and thereby cools the main flow of steam in the steam line. Valves controlled by data from in-line temperature sensors control the relative flows of steam in the main steam line and water entering the main steam line at the attemperator. As the turbine begins to warm up, the relative proportions of steam and water are adjusted so as to supply, over a specified time frame, steam of gradually increasing temperature to the turbine.
However, it is known that there are limits to the amount of water that may be added through an in-line attemperator. Generally it is recognized that the upper limit for addition of water through an attemperator into a turbine steam line is about 15–18 percent of the steam flow for a common in-line attemperator. This may result in less flexibility during startups. A reason for this upper limit is that excessive water added through the attemperator may not vaporize completely. This lack of vaporization results in moisture carryover, where water droplets may travel into the turbine damaging the turbine blades. An example of an operational event where this may occur is the start up of a cold steam turbine with a gas turbine and waste heat recovery steam generator that have been operating at a high load temperature and pressure.
Attemperators of the spray-nozzle type and other types have been utilized in combined cycle turbine facilities. A combined cycle turbine facility generally includes a gas turbine plant, a waste heat recovery steam generator (i.e., HRSG, or boiler), and a steam turbine plant. Each gas turbine in the gas turbine plant is comprised of a gas turbine, an air compressor driven by the gas turbine, and a combustor. In a typical combined cycle system, a fuel/air mixture combusts in the combustor of the gas turbine, expanding to provide mechanical energy to rotate the gas turbine. Electricity is generated by a generator mechanically coupled to the shaft of the gas turbine. The hot gases of the combustion are directed into a waste heat recovery steam generator (i.e., boiler) where water is converted to steam based on energy transfer from the hot gases. This high pressure steam is then used to drive a high pressure steam turbine. The high pressure steam may have a pressure of about 1,000 to 1,500 pounds per square inch (psi), or in some cases up to 2,400 psi, and temperature may be in a range from about 1,000 to 1,050 degrees Fahrenheit. Typically steam at the end of the high pressure turbine then flows serially to drive turbines at lower pressures, such as to an intermediate pressure steam turbine and then to a low pressure steam turbine. The mechanical power from these turbines is converted to additional electricity via one or more generators connected to the turbines. In various designs the steam is reheated between passes through the respective steam turbines. For example, a reheater line in a combined cycle turbine facility may have a pressure between about 300 and 500 psi, and a temperature in a range of about 1,000 to 1,050 degrees Fahrenheit.
In some prior art steam attemperating systems of combined cycle systems, two steam flows having different temperatures are combined, their relative flows being controlled by valving, to provide a desired gradually increasing temperature for warming up a cold turbine. Alternatively, water from one of the condensers is used to cool one of the steam flows using an attemperator. One example of a steam-cooling-steam attemperation device is disclosed in U.S. Pat. No. 3,882,680, issued May 13, 1975 to Durrant and Haller (“the '680 patent). The invention in the '680 patent provides two steam attemperators, one downstream of a superheater outlet, and the other downstream of a reheat outlet. At each attemperator, valves control the inflow of two sources of steam, one cooler than the other. In one disclosed embodiment, there is a header extending laterally, substantially wider than tall, with a manifold extending through the middle of the space defined by the header. The manifold is supplied with lower temperature steam, which passes through spray holes directed downwardly. Higher temperature steam passes from one or more conduits at the bottom of the header, mixes with the lower temperature steam, and exits through a conduit at the top of the header. Appropriate adjustment of valving of the respective steam supplies is stated to provide for controlled and gradual increase in steam temperature for start-ups, controlled shutdowns, and the like.
Other patents that describe steam-cooling-steam approaches include U.S. Pat. No. 4,277,943, issued Jul. 14, 1981 to Silvestri and Kesavan, and U.S. Pat. No. 6,422,022 B2, issued Jul. 23, 2002 to Gorman et al. These patents, and, more generally, all patents, patent applications, patent publications, and all other publications cited herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually set forth in its entirety.
Examples of water being used to cool steam for attemperation include U.S. Pat. No. 3,392,712 issued Jul. 16, 1968 to Lustenader and May, U.S. Pat. No. 3,640,250 issued Feb. 8, 1972 to Costellos and Beckman, U.S. Pat. No. 4,208,882 issued Jun. 24, 1980 to Lopes and Carberg, U.S. Pat. No. 4,372,125 issued Feb. 8, 1983 to Dickenson, U.S. Pat. No. 4,455,836 issued Jun. 26, 1984 to Binstock et al., and U.S. Pat. No. 4,471,620 issued Sep. 18, 1984. An adaptive attemperator is described in U.S. Pat. No. 4,589,255 issued May 20, 1986 to Myers. This patent also provides detailed examples of the steps of cold and warm startups.
In view of the aforementioned limitations of currently used approaches for controlling steam temperature in steam-cycle types of turbine facilities, there is a need for improved systems and apparatuses for cooling steam for start-up of cold turbines and for other purposes.